Cerebral cortex

“ With all that soul-searching and brain-scratching, your mind now ought to be sufficiently elastic and sensitive to attempt a general reorganization of a deranged cerebral cortex—psychokinesis—the goal you've been working toward. ”

“ In ordinary discourse we use the words brain and mind as interchangeable terms, because it is quite evident that where there is lack of intelligence there is deficiency in the brain and where there is a high degree of intelligence there is a correspondingly high development of brain. The immense advance in recent times in our knowledge of the physiology of cerebral process serves more and more to emphasize the entire and absolute dependence of mind on brain, and particularly of the higher processes of mind on the development of the cells and fibres of the cerebral cortex. ”

“ The most exalted organ, however, the condensation of all the other organs, is the gray cerebral cortex. It is the seat of clear consciousness. It works least of all for itself, most of all for the commonwealth—i.e., for the whole organism. ”

“ No simple sensory impulse can, under235 ordinary circumstances, reach the cerebral cortex without first being influenced by subcortical association centers, within which complex reflex combinations may be effected and various automatisms set off in accordance with their preformed structure. These subcortical systems are to some extent modifiable by racial and individual experience, but their reactions are chiefly of the determinate or stereotyped character, with a relatively limited range of possible reaction types for any given stimulus complex. ”

“ Hence the organs of consciousness and control, being organs of adaptation and readjustment of reaction, will be more pronounced the farther the animal scale is followed upward to its crowning species, man. The cerebrum and especially the cerebral cortex may be regarded as the highest expression of the nervous organ of individual adaptation of reactions. Its high development in man makes him the most successful animal on earth’s surface at the present epoch. ”

“ These masses of grey matter, taken together, are the basal nuclei of the brain. Internal to the lenticular nucleus, and between it and the caudate nucleus in front and the thalamus behind, is the internal capsule, through which run most of the fibres connecting the cerebral cortex with the crus cerebri. The capsule adapts itself to the contour of the lenticular nucleus and has an anterior limb, a bend or genu, and a posterior limb. ”

“ The central neurons of the cerebral cortex are, as it were, the tablets, upon which the excitations transmitted thither by the sensory neurons, record the extended neurograms that constitute the physical basis of the concrete imagery of memory and imagination. ”

“ The grey matter of the cerebral cortex is broadly characterized histologically by the perikarya (nerve-cells bodies) which lie in it possessing a special shape; they are pyramidal. The dendrite fibres of these cells—that is, their fibres which conduct towards the perikarya—are branches from the apex and corners of the pyramid. From the base often near its middle arises one large fibre—the axone fibre, which conducts impulses away from the perikaryon. ”

“ This would seem to indicate that the greater mental activities of higher animals depend more on richness in complex associations than on mere increase in number of neurons. The latter, however, is by no means unimportant as may be seen in man, for instance, in whom it is estimated that the cerebral cortex, that is, that part of his brain in which his more complex mental processes transpire, contains some nine billion more nerve cells than does the corresponding region of the brain of an anthropoid ape. ”

“ Other Senses and Localization in the Cortex Cerebri.—Turning now to the connexion between the function of the cortex and the senses other than those of the great distance-receptors just dealt with, even less is known. Disturbance and impairment of skin sensations are observable both in experiments on the cerebrum of animals and in cases of cerebral disease in man. ”

“ Further details of the brain, abundantly illustrated, will be found in the later editions of any of the standard text-books on anatomy, references to which will be found in the article on Anatomy: Modern Human. Das Menschenhirn, by G. Retzius (Stockholm, 1896) , and numerous recent memoirs by G. Elliot Smith and D.J. Cunningham in the Journ. Anat. and Phys. and Anatomisch Anzeig., may be consulted.
Histology of Cerebral Cortex.
The cerebral cortex (see fig. 15) consists of a continuous sheet of grey matter completely enveloping the white matter of the hemispheres. ”

“ The cortical neurone therefore collects impulses in the region of cortex just about its perikaryon and discharges them to other regions, some not cortical or even cerebral, but spinal, &c. One question which naturally arises is, do these cells spontaneously generate their impulses or are they stirred to activity by impulses which reach them from without? The tendency of physiology is to regard the actions of the cortex as reactions to impulses communicated to the cortical cells by nerve-channels reaching them from the sense organs. ”

“ It is on the functions of the fore-brain that interest now mainly focuses, for the reasons mentioned above. And the interest in the fore-brain itself chiefly attaches to the functions of its cortex. This is due to several causes. In man and the animals nearest him the cortex forms by far the larger part of the whole cerebral hemisphere. More than any other part it constitutes the distinctively human feature. ”

“ Through these observations it has been possible to assign certain functions to parts of the physiological mechanism.
In the case of man, both by observation and experiment, “the nervous structure below the hemispheres of the cerebrum has been excluded from the possibility of acting as the immediate physical basis of mental states.” [9] The higher mental processes, which involve the possibility of speaking, calculating, and responding by learned reactions to complex situations, have their correlate in the cortex (the agglomeration of neurones in the cerebral hemispheres) . ”

“ Above, and on either side of the eyes, I saw the shadow of something white, soft, and spongy, in which I fancied I could detect a distinct likeness to a human brain, only on a large scale. There were the cerebral lobes, or largest part of the forebrain, enormously developed and overhanging the cerebellum, or great lobe of the hindbrain, and completely covering the lobes of the midbrain. On the cerebrum I even thought I could detect—for I have a smattering of anatomy—the usual convolutions, and the grooves dividing the cerebrum into two hemispheres. ”

“ They reminded him of the fireflies of the green hells he had been in during the war.
"You are now skirting the convolutions of the cerebellum," his guardian angel told him. "They are electronic tubes which receive sensory impressions and translate them into impulses for cerebration. Here in the cerebellum the bulk of the associations is being evoked; these are then distributed throughout the hemispheres of the cortex or higher brain. Oh I do wish you wouldn't get seasick, Dr. Lee; some of our visitors do, you know; it's those wavy, wavy movements." ”

“ The gray matter of the cerebellum, the medulla, and the cord may receive impressions from the senses and respond to them with movements, but their response is in all cases wholly automatic and unconscious. A person45 whose hemispheres had been injured in such a way as to interfere with the activity of the cortex might still continue to perform most if not all of the habitual movements of his life, but they would be mechanical and not intelligent. He would lack all higher consciousness. ”

“ I would desire this evening to lay down and to enforce a principle that is, I think, not sufficiently, and often not at all, considered in practical medicine and surgery. It is founded on a physiological basis, and it is of the highest practical importance. The principle is that the brain cortex, and especially the mental cortex, has such a position in the economy that it has to be reckoned with more or less as a factor for good or evil in all diseases of every organ, in all operations and in all injuries. ”

“ The greater part of the grey matter is situated as a layer on the outside of the cerebrum, or great brain, where it forms a rind from one twelfth to one eighth of an inch in thickness, known as the cortex. ”

“ Around these cavities is the brain substance, which is made up of two tissues, the white and the gray, the latter [Pg 76] forming the outer part of the brain to the depth of perhaps half an inch, and the white matter forming the rest. The outer or gray part is called the cortex and is largely made up of nerve cells. It might be called the active part of the brain. The white part consists largely of nerve fibers which are given off from the nerve cells and are carried down into the spinal cord. ”

“ For, according to his conception of this science, no other "conditions" but the processes accomplished in this cerebrum, "explain" the existence of thoughts and feelings. Accordingly, the cerebrum is sometimes spoken of as though it "throws off," as a sort of total epi-phenomenon incapable of being analyzed into psychic factors or explained by reference to antecedent psychic conditions, each state of consciousness. With our author, psychology as a natural science, without metaphysics, is wholly cerebral psychology. ”

“ The momentous announcement of modern physiology, that the cerebrum is the organ of consciousness and mental action in man and the higher mammals, is illustrated and confirmed by the pathological study of its diseases. When parts of the cortex are destroyed by disease their respective functions are affected, and thus we are enabled, to some extent, to localize the activities of the brain; when certain parts of the area are diseased, that portion of thought and consciousness disappears which depends on those particular sections. ”

“ A peculiar fact relating to these convolutions is observed by all anatomists: mental [pg 99] development is always accompanied by an increasing dissimilarity between their proportional size.
The cerebral hemispheres may be injured or lacerated without any pain to the patient. The effect seems to be one of stupefaction without sensation or volition. A well-developed brain is a very good indication of intelligence and mental activity. That the cerebrum is the seat of the reasoning powers, and all the higher intellectual functions, is proved by three facts. ”

“ But these volitional, emotional and visual centers are widely spread over the cortical area. About their special connections with one another, and especially with the center of the accessory nerve, under normal conditions, we are much in the dark. And how they could go to work to solve, in any length of time and even partially, the problem of readjusting their functional relations to the new and abnormal conditions, offers a problem as yet quite unanswerable by cerebral physiology. ”

“ He can predict all the movements that will take place as certainly as a chemist knows in advance the reaction he will obtain from mixing certain bodies.
Pathology confirms our conclusion respecting these higher functions of the cerebrum. Loss of cerebral substance, in man, is followed by a weakening of the intellectual powers. They make a very childish mistake who attempt to deal with the physiological materialism of our day by citing the American crow-bar case, or any number of cases of brain-loss unaccompanied by marked intellectual enfeeblement. ”

“ With respect to the organic or physico-chemical conditions of its activity, the brain, then, presents nothing exceptional. If we turn to experiments made upon it by physiologists, we shall find that they have succeeded in analyzing cerebral phenomena in the same way as those of all the other organs. The experimental process usually employed to determine the functions of organs consists in removing them or in destroying them either gradually or suddenly, so as to determine the uses of the organ according to the special disturbances thus caused in vital phenomena. ”

“ In man, the cerebrum presents an immense preponderance in weight over other portions of the brain; in some of the lower animals, the cerebrum is even less in weight than the cerebellum. Another interesting point is the development of cerebral convolutions in certain animals, by which the relative amount of gray matter is increased. In fishes, reptiles, and birds, the surface of the hemispheres is smooth ”

“ From its small size, and from the imperfect development of the convolutions, this brain was, most likely, taken from a very young animal; the cerebral membranes have been removed; the vertex is somewhat flattened; the orbital surfaces have lost their characteristic concavity; the middle lobes have sunk asunder; and the cerebellum has, undoubtedly, been a little displaced backwards. ”

“ There is exaggerated ideation and motor explosion in cases of cortical hyperæmia or congestion. Such might be the case here during the maniacal attacks B 1. But where loss of memory occurs as in the B 2 sub-stage, is it not possible to conceive that the blood-supply may have been shut off in the Broca area, or part of it? The microscope reveals to us only some of the finer blood-vessels and capillaries of the cortex, but it has not yet shown us the most delicate system of channels which bathe the individual cells and fibres in lymph or serum. ”

“ Neither medicine nor physiology opposes the use of the clinical method in cerebral localization. But only circumscribed lesions that have little or no tendency to become general, or to act at a distance by compressing the brain, or otherwise, can come to the aid of our theory. When there is a lesion of the cortical region of the brain which fulfills these conditions, the resulting symptoms may be of two orders—either stimulative or paralytic of the true function. ”